The present invention relates to a method of continuous treatment of grain mash with low temperatures for production of ethanol, wherein enzymes are introduced in two steps for decomposition of the starch to glucose.
For continuous treatment of mash with enzymes, it is necessary to divide the raw material very finely so that the material can be effectively utilized. In the event that the raw material is comminuted coarsely, the enzymatic decomposition cannot be performed sufficiently fast and completely. On the other hand, too fine a grinding is not desirable, on energy considerations. A method of two-step enzymatic production of dextrose is disclosed in German Offenlegungsschrift No. 2,618,131, wherein a wet grinding of a homogeneous suspension of coarse and fine particles is performed in a cycle. In this method, contrary to the present invention, the small particles are again broken into pieces, which does not provide for any advantage inasmuch as it requires high energy consumption.
Another two-step enzyme process is known from U.S. Pat. No. 4,092,434, in which alcohol is produced from grain, wherein the starch is liquefied with addition of alpha-amylase in condition of 75.degree.-85.degree. C. After cooling to 25.degree.-35.degree. C., the beta-glucosidase is added for saccharifying.
In all known continuous low-temperature processes, the grain is mixed in a mixer with 2-3 times water quantity, heated by direct vapor to 60.degree.-90.degree. C., mixed with alpha-amylase and pumped through a contact delay tank or a pipe system. After adjusting the temperature and the pH-value, the amyloglucosidase is added, and the further reaction takes place either in a second contact delay system, or the mash is cooled and pumped directly to a fermentor.
Low-temperature mash processes operate economically because of reduced vapor consumption as compared with the high-temperature processes. The entire energy consumption is, however, not entirely favorable when the costs for the comminution of the grain are taken into consideration. The material must be ground to flour fineness, because otherwise the yield is unsatisfactory. Price-favorable comminution of dried grain is performed in a hammer mill or a wing beater mill. In both systems, the classification is carried out by a built-in sieve insert with opening diameters of 2 mm. The smaller the particles of the starch-containing material, the more complete is the enzymatic decomposition of the starch-containing material particles. On the other hand, it is not meaningful to communite the material to such an extent that all particles exceed a predetermined high limit, for example 0.3 mm, inasmuch as then the natural part of the particles are considerably smaller than required in accordance with the process. The energy and instrument costs increase exponentially with fine grinding and lump-free mixing in water is expensive. A compromise between grinding expenditures and grain size didtribution must be achieved.